'\" et
.TH LISTEN "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
listen
\(em listen for socket connections and limit the queue of incoming
connections
.SH SYNOPSIS
.LP
.nf
#include <sys/socket.h>
.P
int listen(int \fIsocket\fP, int \fIbacklog\fP);
.fi
.SH DESCRIPTION
The
\fIlisten\fR()
function shall mark a connection-mode socket, specified by the
.IR socket
argument, as accepting connections.
.P
The
.IR backlog
argument provides a hint to the implementation which the implementation
shall use to limit the number of outstanding connections in the
socket's listen queue. Implementations may impose a limit on
.IR backlog
and silently reduce the specified value. Normally, a larger
.IR backlog
argument value shall result in a larger or equal length of the listen
queue. Implementations shall support values of
.IR backlog
up to SOMAXCONN, defined in
.IR <sys/socket.h> .
.P
The implementation may include incomplete connections in its listen
queue. The limits on the number of incomplete connections and completed
connections queued may be different.
.P
The implementation may have an upper limit on the length of the listen
queue\(emeither global or per accepting socket. If
.IR backlog
exceeds this limit, the length of the listen queue is set to the
limit.
.P
If
\fIlisten\fR()
is called with a
.IR backlog
argument value that is less than 0, the function behaves as if it had
been called with a
.IR backlog
argument value of 0.
.P
A
.IR backlog
argument of 0 may allow the socket to accept connections, in which case
the length of the listen queue may be set to an
implementation-defined minimum value.
.P
The socket in use may require the process to have appropriate
privileges to use the
\fIlisten\fR()
function.
.SH "RETURN VALUE"
Upon successful completions,
\fIlisten\fR()
shall return 0; otherwise, \-1 shall be returned and
.IR errno
set to indicate the error.
.SH ERRORS
The
\fIlisten\fR()
function shall fail if:
.TP
.BR EBADF
The
.IR socket
argument is not a valid file descriptor.
.TP
.BR EDESTADDRREQ
.br
The socket is not bound to a local address, and the protocol does not
support listening on an unbound socket.
.TP
.BR EINVAL
The
.IR socket
is already connected.
.TP
.BR ENOTSOCK
The
.IR socket
argument does not refer to a socket.
.TP
.BR EOPNOTSUPP
The socket protocol does not support
\fIlisten\fR().
.P
The
\fIlisten\fR()
function may fail if:
.TP
.BR EACCES
The calling process does not have appropriate privileges.
.TP
.BR EINVAL
The
.IR socket
has been shut down.
.TP
.BR ENOBUFS
Insufficient resources are available in the system to complete the
call.
.LP
.IR "The following sections are informative."
.SH "EXAMPLES"
None.
.SH "APPLICATION USAGE"
None.
.SH "RATIONALE"
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIaccept\fR\^(\|)",
.IR "\fIconnect\fR\^(\|)",
.IR "\fIsocket\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<sys_socket.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
